ix PHYLUM ECHINODERMATA 381 



tube-feet are in four rows. Asterias differs from the other genera 

 of the family in having well-developed reticulate dorsal ossicles 

 bearing definite spines. 



The Sea-urchins of which a short description has been given 

 are the genera Strongylocentrotus and Echinus, but the description 

 is sufficiently general to apply to any member of the family 

 Echinidce, to which these genera, with a number of others, belong. 

 The family Ecliinidcc is one of about five families of the sub-order 

 JEctobranchiata, the members of which all differ from the other 

 sub-order Entobranchiata of the JRegularia, or regular Sea- 

 urchins, in the possession of dermal branchiae, and in having the 

 auricles in the form of complete arches. 



The Sea-cucumber (Cucumaria or Colochirus) is a member of 

 the Stichopoda one of the families of the sub-order Dcndrochirotce 

 of the Pedata, or foot-bearing Holothurians. The Dendrochirotw 

 differ from the Aspidochirotm the other sub-order mainly in 

 having arborescent instead of shield-shaped tentacles, and the 

 Stichopoda differ from the rest of the Dendrochirotce in having the 

 ljube-feet arranged in five regular zones. The genus Cucumaria 

 is distinguished from the rest by the ten tentacles with the two- 

 ventral smaller than the others. Colochirus is closely allied to 

 Cucumaria., the principal distinction being the presence in the 

 former of papillae taking the place of tube-feet in certain 

 situations as already noted. 



The Feather Star (Antedon rosacea) is a member of the family 

 Comatulidce, which is distinguished from the four other living 

 families comprised in the order Neocrinoidea, by the absence of a 

 stalk in the adult condition. 



6. GENERAL ORGANISATION. 



General Form and Symmetry. Like the Coelenterata, the 

 Echinodermata are radially symmetrical, the body being capable of 

 division into a series of sub-equal antimeres along a series of 

 radiating planes at right angles to the principal axis. In the 

 majority of existing forms (Asteroidea, Ophiuroidea, and Crinoidea) 

 the radial symmetry is expressed in the external form of the body, 

 which is produced into a number of radially disposed parts, the arms* 

 or rays, arranged around a smaller or larger central disc. But in the 

 Echinoidea the body is sub-spherical, and in the Holothuroidea 

 sub-cylindrical, the radiate arrangement being in these classes 

 indicated externally only by the distribution of the tube-feet, and 

 internally by that of certain of the systems of organs. 



Although, however, the general external form and the arrange- 

 ment of some of the internal organs in the Echinodermata indicates 

 a radial symmetry, it is invariably found that this radial arrange- 

 ment serves to hide a more primitive and more fundamental 



